PTT/PTS is used in many government and military networks to control muting of a telephone handset's transmitter and to signal connected equipment such as radios that their transmitters are to be keyed (i.e., that their transmit functions are to be enabled) When a connection is established between a telephone having a PTT/PTS capability and a radio requiring transmit keying, the radio's transmit function will be turned on when the PTT button is depressed on the telephone, and turned off again when the PTT button is released. PTT reports the state of the phone's transmitter, and PTS requests keying of a remote transmitter. In practice in general usage, PTT and PTS often coincide.
In most existing communications networks such as TDM (Time Division Multiplex), transmission delays are minimal, and the PTT/PTS information can be carried in parallel with the voice using either CAS (Channel-Associated Signaling) or through a separate message-based interface such as ISDN PRI (Integrated Services Digital Network Primary Rate Interface). Since delays are small and predictable, this method of conveying the PTT/PTS information is satisfactory.
However, IP networks generally have higher delay (latency) and greater variability in latency than many conventional legacy networks. Unless steps are taken to prevent it, such networks are prone to what is commonly known as the “shoot/don't shoot” problem. Specifically, a PTT/PTS phone user depresses the PTT button on the handset and speaks the words “don't shoot”, but the distant radio's transmitter is not keyed until the word “shoot” is received, so that only the word “shoot” is broadcast from the radio. This can happen if the PTT/PTS information experiences a greater delay through the IP network than the voice data does. Elimination of this problem in a Voice-over-Internet (VoIP) telephone system would constitute a significant advantage for such a system.